Optical system for ranging by lasers and the like

ABSTRACT

A light splitting and recombining system, protecting observers and apparatus from destructive effects of laser flashes. Light from two paths is combined differentially to reduce, by interference, the light transmitted by the system. In one of the paths there is included a light-bleachable optical element whose transmissivity is a function of incident light energy. The system accepts low energy light of all wavelengths, but substantially stops high energy laser light.

United States Patent lnventor Elias Reisman Orange, Calif.

Appl. No. 719,618

Filed Apr. 8, 1968 Patented Apr. 20, 1971 Assignee Philco-FordCorporation Philadelphia, Pa.

OPTICAL SYSTEM FOR RANGINC BY LASERS AND THE LIKE 10 Claims, 3 DrawingFigs.

US. Cl 356/ 160, 350/163, 356/4, 356/106 Int. Cl 602i 1/28 Field ofSearch 350/ 160, 163; 356/106; 356/4 [56] References Cited UNITED STATESPATENTS 3,455,627 7/1969 Letter 350/ 160 3,470,492 9/1969 Soffer 331/945Primary ExaminerRodney D. Bennett, Jr. Assistant Examiner.loseph G.Baxter Attorney-Carl H. Synnestvedt ABSTRACT: A light splitting andrecombining system, protecting observers and apparatus from destructiveeffects of laser flashes. Light from two paths is combineddifferentially to reduce, by interference, the light transmitted by thesystem. In one of the paths there is included a light-bleachable opticalelement whose transmissivity is a function of incident light energy. Thesystem accepts low energy light of all wavelengths, but substantiallystops high energy laser light.

OPTICAL SYSTEM FOR RANGING BY LASERS AND THE LIKE In systems where laserlight is used or encountered, for instance in the determination ofdistances by laser ranging" methods, serious dangers are caused bypossible flashes of enormously intense light, coming or returning fromremote areas and which can enter an observers eye, or a sensitiveinstrument, with irreparable harm. The perilous flashes can come fromvarious sources, including not only remote lasers operated by others,for example by enemies, but also strong reflections of the user's ownlaser beam.

Heretofore it was proposed to counteract such dangers by using a narrowband filter. Particularly a filter rejecting light of 6,943 A wavelengthcould be used to avoid danger from ruby lasers. Such proposals had thedrawback that they impaired the users observation of his own operations,especially those carried out at the wavelength of the ranging instrumentitself.

It has also been suggested to interpose material in the line of sightwhich would blacken or become opaque on exposure to high intensitylight. These proposals also have various drawbacks, including relativeslowness of the blackening process, as well as the danger of evaporationand loss of more rapidly blackening elements.

In contrast to these and similar proposals of the past, the inventionuses a modified form of a light splitting, recombining andinterference-inducing system, wherein it incorporates, in one of theseveral light paths, a known light bleachable element, not a lightblackening element. Advantageously the transmissivity of the element isa function of incident light energy. The new combination, which will nowbe described, achieves in response to a laser flash an ,enonnousreduction in system transmissivity, greatly exceeding the limitedincrease in transmissivity of the light bleachable element itself.

In the drawing appended hereto FIG. I is a schematic view of a simpleembodiment of the new combination. FIGS. 2 and 3 are similar views,showing the principal components of a somewhat modified and preferredsystem.

Referring to FIG. I, the ranging or tracking instrument T receivesordinary background light BL and may also receive laser light LL. In theinstrument, the incident light enters a system which basicallyconstitutes a Michelson interferometer, and wherein first and secondmirrors M-1 and M-2 are at right angles to one another while an obliqueglass mirror, installed to face both minors, has a semireflectivesurface M3 at 45 to M-l and M-2. Light BL, LL, incident on surface M3,is split into beams I, II, advantageously of equal energy, which aredirected to the two mirrors. By means of micrometer screw A, thedistance from full reflector M-l to oblique semireflective surface M-3is adjusted to be such, by comparison with the distance from m-3 to thesecond full reflector M-2, that maximum interference occurs. In thesimple embodiment shown, and when disregarding transmission times inM-3, B and C and phase shifts at M-I, M-Z, this condition occurs if Dequals 1),:tn It 4, where A is the wavelength of laser light LL, n is anodd integer, and D,, D are, respectively, the center distances from M6to M and M By such adjustment the interferometer is optically tuned tothe laser light, to obtain differential interference.

According to the invention a disc of light bleachable material B isdisposed in light path I1, between the semireflective surface and one ofthe mirrors, for instance as shown between M-2 and M-3. This material,which is also referred to as light fading, can for instance consist ofthe substance identified as RG-Itl glass," which comprises selenium andcadmium sulfide. It is made by Jena Glaswerk Schott & Gen., Mainz,Germany and is distributed by Fish-Schuman Corporation of New Rochelle,New York. Instead of this substance a solution of metal phthalocyaninescan also be used. Both substances have the characteristic that theirlight transmission coeflicient increases to some limited extent, withthe intensity of the incident light, within fractions of a nanosecond,and that the for instance by Georges Bret and Francois Gires, ComptesRendus, Academic des Sciences, Vol. 259 pp. 3469 3471 (1964) and AppliedPhysics Letters, Vol. 4, pp. 175, 176 (1964). By means of the newcombination of elements the limited increase in transmissivity of theelements leads to a much greater reduction of transmissivity of thetotal system.

Preferably, a compensator C for element B is disposed in light path I.This compensator can be a neutral density filter having a transmissionequal to that which light bleachable disc B has in its state of maximumtransmission. Substantially complete elimination of the danger caused bylaser light can be achieved by this arrangement, while the utilizationof nonlaser light, including light at laser wavelength, remains almostentirely undisturbed.

If the new instrument is utilized as part of a ranging device, it isrigidly connected with laser L, as is schematically suggested by thebracket interconnecting the several devices. For purposes ofillustration FIG. 1 assumes that light LE, generated by laser L andcollimated by laser optics L0, is projected to some distant object, notshown, and is reflected therefrom as light LL, shown on the left side ofthe FIG. This light is received in instrument T.

In this instrument, a ranging unit R, per se known in the art, andincluding for instance a well known photodiode and oscilloscope circuit,normally receives light reflected by mirror M-l, and a considerablysmaller amount of light from the second mirror system M-Z, due to thenormal low transmissivity of disc B. The output can also be observed byhuman observer H, with the aid of insertable mirror M-4.

Any laser light LL received in the instrument, which may be generatedfor instance by laser generated flashes of intense laser light emissionLE, reflected from remote objects are attenuated by the instrument, asthe interference system is particularly tuned to this light and providesmaximum differential interference as to such light. However, when highintensity light of this kind is received, the transmissivity of lightbleachable disc B rises; the disc becomes more transparent. More of thelight incident on this disc then reaches second mirror M-2, and returnstherefrom (again through B) to semireflective surface M-3. Here it isrecombined with the light that returns from mirror M-l. Interferencethen occurs between substantially equal portions of laser light flashes,returning from MI, and M-2 and recombined at M-3, thereby effectingsubstantially total cancellation in the output of the system.

In this connection it will also be understood that laser emission is ofhigh spectral purity at a certain wavelength A. Pursuant to propertuning, as indicated above, substantially identical intensities ofspectrally substantially identical light flashes are subjected todestructive interference, the phase of one portion differing from thatof the other by exactly one-half wavelength of the specific laseremission LE. The result is that maximum energy laser light LL, reachingthe instrument, produces a minimum energy light output, approaching zeroin output system R, H.

Accordingly, this system sees substantially only background light BL,and any low intensity and nonperilous laser light returning for instancefrom objects of low reflectivity in the distance. These latterconstituents, however, are fully observable, along with background lightBL. In this respect the new system is distinctly superior to the laserlight filtering systems of the past.

As a resume, the operation of the new system can be described as followsfor the two principal conditions that it encounters, i.e., the lowintensity condition of incident light and the high intensity conditionof such light. Assuming in the first place that the instrument receivesonly light of an energy expressed as unity (1.000), semireflector M-3reflects onehalf of this light (0.500) to mirror M-I, and thatcompensator C attenuates this to 0.315 on a double pass, it will then befound that half of 0.315 that is, 0.1575 is transmitted into outputsystem RH. The second half of the incident low-energy variation isreversible and repeatable, as has been pointed out unit of light (0.500)is transmitted to, and part of it returned from, mirror M-Z. Nonnallythere is a return of substantially less than 0.3 15 upon a doublepassthrough bleachable filter B. When this filter is made of R640 and of 40mil thickness it transmits 0.016 out of 1.000 on a double pass, that is,0.008 out of the incident 0.500. One-half of this 0.008 is finallyreflected at M-3, so that 0.004 reaches R, H from M-2. Since the energyfrom M-2, as noted, is transmitted at a phase shift of one-halfwavelength relative to that from M-l, its energy is deducted from thegreater energy that comes from M-l. Thus the net output of the system is(0.l575" or about 0.112, that is, 11.2 percent of the unit input in thelow intensity condition.

Referring now to the high intensity condition of the system: it may beassumed that at this time an input of 1000 units is incident on M-3,whereof 500 goes to and 315 returns from M-l to M-3 and 157.5 to R. Theother 500 goes to B, and in view of the high intensity condition of thiselement, instead of the former 0.016 an increased double-pathtransmission of 0.63 occurs. Therefore M-3 now receives 315 units fromM-1 and the same amount from M-Z. Again, the return from M-2 arriveswith 180 phase difference, and except for minor discrepancies caused bysuch factors as different reflectivities of the difierent mirrors, whichcan be additionally compensated in known ways one-half of each return,or +l57.5 and l57.5, now reaches the output system, so that the systemnow transmits substantially percent of the 1000 units input.

The combination of reflective, semireflective and lightbleachableelements M1, M-2, M-3, B, which characterizes this invention, may beprovided conveniently and most effectively in form of a compositeprismatic unit, shown in H0. 2. This unit comprises two uniform 90 glassprisms (3-1, (3-2, combined to form an approximately cubical body. Eachprism has, in crom section, the form of an isosceles, right-angledtriangle One prism G-l has a first mirrorized surface M-l! on one of itssides, this surface desirably having its reflectivity so adjusted as tomatch the loss of transmission equal to that of the bleachable elementat maximum transmission. The other prism 6-2 has the light bleachablematerial, as a layer B on the corresponding side. It has been foundparticularly effective to make this layer 40 mil thick and to placereflective surface M-12 on the exposed side of this layer. The twomirrors can be formed by evaporating a suitable dielectric onto the endsurfaces of the prisms. The base diagonals or hypotenuse surfaces of thetwo prisms are in contact with one another, one of these surfaces havinga lightsplitting and in the simplest case semi-reflective coating M-13thereon. in order to provide the required interference in light output0, prism G4 can be minutely adjusted relative to G-2, along M43, asindicated by full and broken lines at the top and left side of the unit.

Light input Ll, incident on the center of a semireflective surface M-13and substantially fully reflected at M-1 1, traverses and retraversesone-half of the side length of the cube minus one quarter of A, beforeit arrives at the center the second time. When light bleachable coatingB is transparent, a substantially equal portion of the intense light,incident at this center and then returning from M-l2, has traversed andretraversed exactly one-half of the side length of the cube. Thus aphase difference of 11-2 or 180 degrees exists between equal lightportions of wavelength A, recombined in the center of the cube. Thelight output 0 then approaches or equals zero, particularly when glassof good optical quality is used for the cubes, avoiding trouble due toscattering of light.

The new unit produces these remarkable efiects virtuallyinstantaneously. The light bleachable material changes from high(typically 0.63) to low (typically 0.010) transmissivity, and back,within time cycles of fractional nanoseconds. (In fact, some of the moreconventional uses of the material rely (that is, becomes transparent atfilter B and causes full interference at R) in times of the order of afew tenths of a nanosecond, when triggered bya high intensity lightpulse. The less critical time delay needed by the new system to open"(that is, to become low-transmissive or opaque at B and no longer tocause significant interference at R) amounts to a few nanoseconds afterthe high intensity of the pulse is over. A single unit of the type shownin FIG. 2 is able, as a result, to reduce the laser energy, transmittedthrough the system, by a major amount.

Still further reduction becomes possible by cascading such units, asshown in FIG. 3. It is only necessary to interpose light beamconcentrating optics LC-for instance, as shown, a focusing lens followedby a collimating lens between the first and second prism units P-1, P-2,in order to make sure that the output of P-l, while reduced inintensity, still is sufficient to trigger the operation of P-Z.

A cascade arrangement similar to that of HQ 3, can also be used toprotect the user against lasers operating at different wavelengths, suchas ruby and neodymium lasers. Light concentrating optics LC are notneeded in this case.

1 claim:

1. A system for transmitting light and for automatically blocking highintensity light, such as light from a laser, said system comprising: anelement for splitting incident light into first and second lightportions and directing the same into first and second paths,respectively; first and second reflectors for reflecting the first andsecond light portions back to said element and for recombining themthereby; and a light control element, disposed in one of said paths, thelight transmissivity of which element rises and falls with the intensityof the light incident thereon.

2. A system as described in claim 1, including means for spacing thereflectors from the semireflector so as to tune the system to thewavelength of incident laser light.

3. A system as described in claim 1, additionally including compensatormeans for causing a transmission loss equal to that caused by said lightcontrol element at maximum transmissivity thereof. said compensatormeans being disposed in the path not including the light controlelement.

4. A system as described in claim 1 wherein said light control elementis a disc or layer of a compound of selenium and cadmium sulfide.

5. A system as described in claim 3 wherein said light control elementcontains a solution of metal phthalocyanine.

6. A system as described in claim 1 wherein said light control elementcomprises a filter of material bleachable by light of high intensity.

7. A protective device for use in light receiving apparatus, said devicecomprising a pair of right angle prisms, each having a pair ofperpendicular faces and a hypotenuse face, said prisms being mirrorizedon said perpendicular faces, and substantially joined along saidhypotenuse faces, each of the latter faces being semireflective, and alight bleachable filter between said hypotenuse faces of said prisms andone of said perpendicular faces.

8. Adevice as described in claim 7 including an additional pair ofprisms similar to the described pair and cascaded therewith.

9. A device as described in claim 8, also including means for focusingand then collimating the light output of the first pair of prisms beforeapplying it to the additional pair of prisms.

10. A device as described in claim 8, wherein one of said pairs ofprisms is optically tunable to one laser frequency, and the other pairof prisms, to another laser frequency.

1. A system for transmitting light and for automatically blocking highintensity light, such as light from a laser, said system comprising: anelement for splitting incident light into first and second lightportions and directing the same into firsT and second paths,respectively; first and second reflectors for reflecting the first andsecond light portions back to said element and for recombining themthereby; and a light control element, disposed in one of said paths, thelight transmissivity of which element rises and falls with the intensityof the light incident thereon.
 2. A system as described in claim 1,including means for spacing the reflectors from the semireflector so asto tune the system to the wavelength of incident laser light.
 3. Asystem as described in claim 1, additionally including compensator meansfor causing a transmission loss equal to that caused by said lightcontrol element at maximum transmissivity thereof. said compensatormeans being disposed in the path not including the light controlelement.
 4. A system as described in claim 1 wherein said light controlelement is a disc or layer of a compound of selenium and cadmiumsulfide.
 5. A system as described in claim 3 wherein said light controlelement contains a solution of metal phthalocyanine.
 6. A system asdescribed in claim 1 wherein said light control element comprises afilter of material bleachable by light of high intensity.
 7. Aprotective device for use in light receiving apparatus, said devicecomprising a pair of right angle prisms, each having a pair ofperpendicular faces and a hypotenuse face, said prisms being mirrorizedon said perpendicular faces, and substantially joined along saidhypotenuse faces, each of the latter faces being semireflective, and alight bleachable filter between said hypotenuse faces of said prisms andone of said perpendicular faces.
 8. A device as described in claim 7including an additional pair of prisms similar to the described pair andcascaded therewith.
 9. A device as described in claim 8, also includingmeans for focusing and then collimating the light output of the firstpair of prisms before applying it to the additional pair of prisms. 10.A device as described in claim 8, wherein one of said pairs of prisms isoptically tunable to one laser frequency, and the other pair of prisms,to another laser frequency.